Equalizer winding for five-legged three-phase cores



May 6, 1952 K. D. BEARDSLEY 2,595,753

EQUALIZER WINDING FOR FIVE-LEGGED THREE-PHASE CORES Filed Dec. 31. 1949 F" .I. I6 2 Kenneth D. Beardsley,

His Attorn ey.

Patented May 6, 1952 EQUALIZER WINDING FOR FIVE-LEGGED THREE-PHASE CUR-ES Kenneth D. Beardsley, Pittsfield, Mass., assignor to General Electric Company, a corporation of New York Application December 31, 1949, Serial No. 136,235

2 Claims. (01. 323-50) This invention relates to induction apparatus and more particularly to three-phase transformers of the five-legged core type in which Y-Y connections are employed. The use of five-legged cores permits the construction of transformers of reduced height as compared with transformers employing other types of cores. Such transformers, therefore, have found extensive use in mining applications and the like in which head-room is limited and apparatus of minimum height is desired. It has been found, however, that when Y--Y connections of primary and secondary windings are employed with transformers having five-legged cores substantial displacement of the neutral point generally occurs, with resultant unbalance of the transformer voltages.

It is accordingly a principal object of the present invention to provide an improvement in induction apparatus of the five-legged cor type, whereby neutral displacement is substantially prevented when Y--Y connections are employed therein. In accordance with my invention I provide equalizer windings on th outside legs of five-legged cores to reduce to a minimum neutral-shift due to the difference in phase of the fluxes which exist in the outside legs.

My invention will be better understood from a consideration of the following description taken in connection with the figures of the accompanying drawing, and its scope will be pointed out in the appended claims. In the drawing Fig. 1 is a schematic cross section of a transformer of the five-legged core type having a wound strip-steel core and including equalizer windings in accordance with my invention; Figs. 2a and 2b are vector diagrams illustrating, respectively, neutralshift in the fluxes in a five-legged core without the use of equalizer windings and the corresponding fluxes without neutral-shift; Fig. 20 gives the algebraic relationships of the fluxes shown in Figs. 2a and 217; Fig. #3 is similar to Fig. 1, except with modified equalizer windings and connections; and Fig. 4 is similar to Fig. 1 except that the core shown therein is of conventional laminated construction.

Referring now to Fig. 1 there is shown a threephase transformer having a five-legged core I, primary windings 2 and secondary windings 3. Core I is of the wound strip-steel type comprising a plurality of separate coils 4, 5, 6 and 1. Coils 5 and 6 may be termed center coils while coils 4 and I are end coils, Corresponding side portions of coils 4 and 5, coils 5 and 6, and coils 6 and 1, respectively, are placed in contact to form the main center legs A, B and C of the transformer.

The remainin side portionsof coils i and 1 form the auxiliary outside legs 6 and 9 of the transformer.

Primary windings 2 and secondary windings 3 are wound on main legs A, B and C, and are ar ranged for YY connections. In other words corresponding ends of windings 2 are commonly connected to 'form a neutral point N2, while corresponding ends of windings 3 are commonly conne-cted to form a neutral point N3. The opposite or free ends of windings 2 form primary connec tions 2 to the transformer, while the corresponding ends of windings 3 form secondary terminals 3' of the transformer. It will be understood that the terms primary and secondary are em ployed primarily for purposes of distinguishing between the two sets of windings and either windings 2 or windings 3 may be input windings of the transformer while the other windings are the output windings thereof.

A pair of auxiliary or equalizer windings iii and H are provided on outside legs 8 and 9 and are oppositely wound thereon. Winding H3 terminates in terminals l2 and it, while winding ii terminates in corresponding terminals i l and 5. Terminals I2 and I4 are interconnected and terminals l3 and I5 are similarly interconnected. A switch It is shown in the line interconnecting! terminals I3 and I5 for the purpose of illustrating the efiect of placing equalizer windings iii and i l in operation, as will be described hereinafter.

In operation three-phase voltages are impressed on either windings 2 or 3 and a plurality of fluxes are established in the various legs of core I. The fluxes in individual coils 4-1 are designated 47, respectively, and are represented in Fig. 1 by arrows having similar designations. While in dividual fluxes 4'1 exist in coils 41, the net fluxes 4m, B and c existing in main legs A, B and C, which are representative of the voltages in windings 2 and 3, are th vector differences of the component fluxes existing in the individual coils forming legs A, B and C. More particularly, A is the vector difference of 4; and 5. Similarly, 4m is the vector difference of 5 and (be, and e is the vector difference of 9156 and m. The subtraction of the various fluxes is shown algebraically in Fig. 20.

Referring now to Fig, 2a, there is shown a vector diagram illustrating the relationship 01 the various fluxes in core I without the effect of equalizer windings ID and II, or with switch l5 assumed to be open. It will be seen that fluxes rm and 7 are equal but not in phase. Accordingly fluxes and c are equal in magnitude and symmetrical with respect to the vertical axis, but B is smaller due to the shift of the neutral from N11) to N'qi. It can be shown that such neutral-shift is due to a difference in phase between fluxes 4 and 457. By forcing fluxes 1; and 57 to be in phase, therefore, the neutral-shift may be substantially eliminated or reduced to a minimum. In accordance with my invention I accomplish this result by the use of equalizer windings l and II which are provided with equal numbers of turns and are oppositely wound on outside legs 8 and 9. If it is assumed that equalizer windings Ill and I l are connected in series relation by closing switch Hi, the vector relationship shown in Fig. 2b is obtained wherein fluxes :1; and 7 are equal and in phase. It will be seen that the component fluxes (p4 and 4m are in phase and that B is substantially equal in magnitude to A and c. Accordingly there is substantially no neutral shift.

In the arrangement of Fig. 1 windings I8 and ll are shown oppositely wound on legs 8 and 9 with corresponding terminals connected together. It will be obvious, however, that these equalizer windings may be wound in the same direction and cross-connected. Such an arrangement is shown in Fig. 3 wherein equalizer winding 19 is shown wound on leg 8 in the same direction as in Fig 1. Equalizer winding ll, however, is wound on leg 9 in the same direction as winding 10 and not in the opposite direction with respect thereto as winding H is wound in Fig. 1. While, in the arrangement of Fig. 1, core I is shown as being of the wound strip-steel type, it will be obvious that my invention applies equally well in connection with other types of five-legged cores as, for example, a core of the conventional laminated type as shown in Fig. 4.

It will be noted that the equalizer windings provided by my invention are wound on outside or auxiliary legs which otherwise are devoid of windings and are, therefore, easily provided therewith. Moreover the use of such windings does not appreciably increase core losses or create other undesirable effects. From the foregoing it will be seen that my invention thus provides a simple and inexpensive structure in three-phase induction apparatus employing five-legged cores and having YY connections, to eliminate or reduce neutral-shift to a minimum and thus provide inherent neutral stability therein.

While I have shown and described preferred embodiments of my invention, it will be understood that my invention may well take other forms and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In three-phase induction apparatus, a magnetic core of the five-legged type having three main center legs and a pair of auxiliary outside legs, main windings wound on said main legs and including primary and secondary windings connected in Y--Y relationship, said primary winding being connected to a three-phase alternating current source, and a pair of equalizer windings wound on said auxiliary legs and connected in series opposed relation with each other to reduce to a minimum neutral-shift in the fluxes in said main legs and in the voltages in said main windings.

2. In three-phase induction apparatus, a magnetic core of the five-legged type having three center legs and a pair of outside legs, main windings wound on said center legs and including primary and secondary windings connected in YY relationship, said primary winding being connected to a threephase alternating current source, and a pair of equalizer windings, one of said equalizer windings being wound on one of said outside legs and the other of said equalizer windings being wound on the other of said outside legs, said equalizer windings being oppositely wound with respect to each other and connected in series relation with each other to reduce to a minimum neutral-shift in the fluxes in said center legs and in the voltages in said main windings.

KENNETH D. BEARDSLEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,265,700 Outt Dec. 9, 1941 FOREIGN PATENTS Number Country Date 461,139 Great Britain Feb. 10, 1937 484,451 Great Britain May 5, 1938 108,080 Germany Aug. 3, 1943 

